When the human heart contracts, it accelerates blood towards the head. The blood travels over the aortic arch, changing direction towards the feet. These accelerations produce equal and opposite forces on the body. A graph of the motion of a freely suspended body due to these forces is called a ballistocardiogram. Ballistocardiagrams have been studied extensively for medical diagnostic purposes. The minimum, resting amount of impulse is relatively constant between persons. The typical impulse per heartbeat is of the order of 5 Newton-seconds, with one heartbeat per second. Smaller individuals have a smaller impulse per heartbeat but more heartbeats per second. Most of the power is smoothly distributed over the 4 to 7 Hertz frequency range.
The shockwave generated by a beating human heart couples to any object with which the human body is in contact. Thus, the level of vibration produced by a beating heart is detectable by a geophone or similar sensor, even when a human is concealed in a large semi-tractor trailer.
Recognizing this concept, an early vehicle inspection system was developed which analyzes a histogram of vibration power levels over time to determine if there is a consistent minimum level. With systems of this type, as shown by U.S. Pat. No. 4,415,979 to Hernendez, ambient seismic or wind noise must be low to permit the system to operate rapidly and effectively. If seismic or wind noise is substantial, a detection decision with this prior system may require many minutes, and no conclusion may be reached in any reasonable amount of time. In some instances, false alarms may be produced.
Subsequently, an enclosed space detection system (EDS) was developed by Lockheed Martin Energy Systems, Oak Ridge, Tenn. and engineers for the Department of Energy, Oak Ridge Tenn. This system uses sophisticated wavelet transforms in combination with fast-Fourier transforms to isolate a detected heartbeat signal from other vibrations. A system operator must choose from a plurality of vehicle icons the one which most closely resembles the configuration of a vehicle to be inspected, and then a plurality of sensors unique to that vehicle configuration must be attached to the vehicle. If the wrong number of sensors are attached to a specific vehicle type or the sensors are not properly oriented, the system will not operate properly. Even when all sensors are properly attached to the vehicle, data may need to be retaken a plurality of times due to spurious ambient vibrations before an accurate indication relative to the presence or absence of a concealed human is obtained. High winds and ground vibration can result in false positive indications.
Obviously, systems using only vehicle mounted sensors can quickly and accurately determine whether or not a person is concealed within the vehicle under quiet ambient conditions by simply providing an indication as to whether sensed vibrations of the vehicle are above or below a threshold. However, vehicle vibration is normally also generated by wind and/or seismic vibration. Thus, sensed high vibration levels may not indicate that a concealed person is present, because the vibration may be due to these other sources.
The level of interference produced by the wind cannot be reliably predicted from the wind speed. A 15 mph wind may not cause a problem, while a 3 mph wind can cause consistent false alarms with solely vehicle mounted sensor systems. The level of wind interference also depends on the shape of the vehicle and the direction of the wind. A long, straight vehicle surface perpendicular to the wind direction will produce a larger vehicle vibration than will be produced by a more streamlined shape. The mechanism of vibration generation is the same as that which produces audible whistling at higher wind velocities. The frequencies for heartbeat detection are, as previously indicated, in the 4 to 7 Hz range. Wind of relatively low speed can sometimes produce substantial vehicle vibrations at these frequencies. The amplitude and frequency of wind vibrations also depend on the acoustic properties of cavities in the vehicle and other mechanical vehicle details.
For some applications, wind noise can be eliminated by placing the vehicle in an enclosure provided by a shelter or wind screen. Isolation from seismic noise would still not be present however, for while vehicle suspensions do provide substantial isolation from the ground, elements such as heavy truck traffic cause substantial seismic ground vibration levels.
It is a primary object of the present invention to provide a novel and improved apparatus and method for human presence detection in vehicles which deletes corrupted vehicle vibration data with simple thresholds on seismic and/or infrasonic power.
Another object of the present invention is to provide a novel and improved apparatus and method for human presence detection in vehicles which provides a stable measure of vehicle vibration, independent of vehicle damping, with the use of constant thresholds to eliminate the effects of high, disturbing forces.
Yet another object of the present invention is to provide a novel and improved apparatus and method for human presence detection in vehicles which employs a vehicle contact sensor to sense vehicle vibration levels and a ground contact sensor to sense seismic vibrations. The sensed seismic vibrations are compared to a threshold so that corrupted vehicle vibration data can be deleted.
A further object of the present invention is to provide a novel and improved apparatus and method for human presence detection in vehicles which includes a vehicle contact sensor to sense vehicle vibration levels and an infrasonic power sensor to sense infrasonic vibrations in the air near the vehicle within a desired frequency range and to compare the sensed infrasonic vibrations within the frequency range to a threshold so that corrupted vehicle vibration data can be deleted.